1. Field of the Invention
The present invention generally relates to a binder system for use in the field of forming ceramics or ceramic bodies and a method of producing ceramics or ceramic honeycomb bodies utilizing such a binder system. More particularly, the invention is concerned with a binder system that contains a low molecular weight oil-based non-solvent and the use of the binder system in a method for producing ceramic honeycomb bodies.
2. Discussion of the Related Art
Binders and binder systems useful for the manufacture of products from powdered materials, e.g., from particulate ceramic materials, must meet a number of requirements. For example, the binder and binder systems must be compatible with the ceramic material such that a flowable dispersion comprising a relatively high loading of the ceramic material in the binder may be provided. In addition, the xe2x80x9cgreenxe2x80x9d preform produced by shaping the dispersion of ceramic powder in the binder should have reasonable strength such that it can be handled. For desirable xe2x80x9cburnoutxe2x80x9d or removal of the binder, the binder should be removable from the shaped ceramic part without incurring distortion or breakage of the part. Furthermore, the binder-free preform should have at least a minimum level of strength, yet be sufficiently free of binder residues that defect-free consolidation is readily achievable.
The formulation of binders meeting these requirements is complex and a large number of different binder formulations have been disclosed in the prior art. Recently, cellulose ether binders have been favored for use in the forming articles of various shapes; i.e., honeycomb substrates. The mixtures are intimately blended and homogeneous and result in the green body having good integrity in size and shape, as well as uniform physical properties. In addition to the binders, these powder mixtures typically include certain organic additives, including for instance, surfactants, lubricants, and dispersants and function as processing aids to enhance wetting thereby producing a uniform batch.
Recently, there has been an increase in the demand for thinner walled, higher cell density cellular structures, complex shaped product, and products having a large frontal area. Thin walled and complex shaped products that are produced, utilizing the current binder technology, i.e., cellulose ether binders, are extremely difficult to handle without causing shape distortion because of the low strength of the xe2x80x9cgreenxe2x80x9d preform. One solution/recent trend in the extrusion technology, especially for multicellular honeycomb bodies, comprised of highly filled ceramic powder mixtures, is to extrude a stiffer body without causing proportional increase in pressures. However, attempts to extrude stiffer ceramic batches with the current batch components; i.e., use of the aforementioned cellulose ether binder coupled with the lowering the amount of water and/or including additives such as sodium tallowate or sodium stearate, have been largely unsuccessful because of the higher extrusion pressures resulting from collision of finer particles, and the abrasiveness of the materials involved.
Another attempted solution is to utilize rapid-setting techniques; i.e., solidifying the cell walls of the honeycomb quickly after forming, therefore ensuring that the dimension of the greenware will not be altered in subsequent cutting and handling steps. Prior rapid stiffening methods involve time-delayed stiffening using rapid set waxes as disclosed, for example in U.S. Pat. No. 5,568,652, and/or applying an external field such as an RF field at the die exit. Although these rapid-stiffening methods involve the extrusion of soft batches, which historically, for highly filled ceramic mixtures, have lead to better extrusion quality, these methods have not been overly successful for thin walled cellular structures.
A recent solution disclosed in co-assigned U.S. patent application Ser. No. 60/069,637 (Chalasani et al.) involves a powder mixture, for forming honeycomb structures, that includes inorganic powder materials, binder, solvent for the binder, surfactant, and a component which is non-solvent with respect to the binder, solvent and powder materials. This powder mixture is mixed, plasticized and shaped to form a green ceramic preform body having improved wet green strength and is thus especially suitable for use in the processing of thin walled honeycomb structures. Furthermore, Chalasani discloses a preferred aqueous binder system mixture that includes water, cellulose ether and a hydrophobic non-solvent.
While this Chalasani reference provides significant advances in the capability of the art to form complex, thin-walled ceramic honeycomb bodies through extrusion, the inclusion of this non-solvent in the powder, e.g., light mineral oil, results in additional complications in the xe2x80x9cburnoutxe2x80x9d or removal of the binder. Specifically, it is difficult to remove the binder components from the shaped ceramic part without incurring distortion or breakage of the part. Because of the reduced strength of the thin-wall ceramic honeycomb bodies and the corresponding increase in the dimensional changes during binder removal due to the exothermic nature of the removal of the oil, special considerations in the firing of the ceramic honeycomb must be undertaken to avoid cracking of the ceramic body. Specially designed kilns, apparatus for volatile removal, reduced oxygen containing atmospheres and increased, complicated firing cycles are among the numerous means that have been employed to reduce the differential shrinkage and high cracking frequency experienced during the firing of thin-walled ceramic honeycomb bodies which incorporate the aforementioned binder.
In light of the foregoing inconveniences experienced in the art, there remains a need to develop a binder system which permits a thin-walled ceramic body to be formed and fired into a desired ceramic article without high differential shrinkage and incidences of cracking or defects and which can be quickly and easily removed from the ceramic body.
It is therefore an object of the present invention to provide a binder system capable of use in forming ceramic or other inorganic honeycomb bodies which results in a sufficiently high wet strength of the formed green body with at least a portion of the binder system capable of being removed without generating a large exothermic reaction as a result of removal during the firing process, thereby reducing the incidences of cracking and the amount of differential shrinkage during firing.
The above object of the invention may be achieved according to the invention that provides a binder system for use in the formation of ceramic or other powder-formed greenware, comprising the essential components of a binder component, a solvent for the binder component, a surfactant component, and a component that is non-solvent with respect to the binder and solvent components. The non-solvent component exhibits a lower viscosity than the solvent when containing the binder and comprises a low molecular weight oil having a 90% recovered distillation temperature range of between about 220 to 400xc2x0 C.; i.e., low molecular weight oil.
According to another embodiment of this invention, there is provided a moldable powder batch comprised of an inorganic powder component, consisting of a sinterable inorganic particulate material, and the aforementioned binder system.
According to another embodiment of the invention there is provided a method for forming and shaping plasticized powder mixtures comprising compounding an inorganic powder component consisting of a sinterable inorganic particulate material mixture and the inventive binder system and thereafter plasticizing the components to form a plasticized mixture and then shaping the plasticized mixture to form a green body.
According to a final embodiment of the invention there is provided a method of making ceramic articles comprising the aforementioned steps of forming and shaping the inorganic powder and binder system composition and thereafter involving the additional step of heating this ceramic green body, having the desired shape, so as to remove the binder system and to fire the formed ceramic body thereby resulting in a fired ceramic body.
An advantageous feature of the binder system is that it is useful for producing cordierite honeycomb structures having thin walls and a large number of cells. Specifically, the resulting so-formed wet green body exhibits a high degree of stiffness necessary to avoid slump in ceramic honeycomb structures with very thin walls; those having a thickness less than 150 xcexcm. An additional benefit of this invention is that firing cracks associated with previous binder systems are reduced, while still maintaining the extrusion benefits of the previous binder systems. Specifically, the low molecular weight oils, utilized in the present invention, exhibit a lower exothermic intensity associated with their removal when compared to previously utilized oil based non-solvent components.